Micro-Electro-Mechanical-System (MEMS), also referred as micro-system or micro-machine, is a high-tech type of electro-mechanical system. MEMS devices are developed based upon microelectronic technologies and semiconductor manufacturing technologies. MEMS devices are formed by the combination of the thin film technology, the photolithography technology, the bonding technology, the silicon micromachining technology, the non-silicon micromachining technology, and the precise micromachining technology, etc. MEMS devices include a plurality of types of devices, such as MEMS sensors and MEMS actuators, etc. Among MEMS devices, the MEMS sensors have been widely used in the protection systems of automobile, inertia control system, home appliance, and various kinds of apparatus, and the systems of many other industries, scientific and engineering systems, etc. Specifically, the MEMS sensors include accelerating sensors, pressure sensors, vibrational gyroscopes, and micro-relays, etc.
A MEMS device needs a cavity to isolate the effects from external environments. To form such a cavity, it needs to bond the substrate of the MEMS device with a cover plate. The bonding process includes the silicon-silicon direct bonding process, the eutectic bonding process, the silicon alloy bonding process, or the glass frit bonding process, etc. Among such bonding processes, the eutectic bonding process has certain advantages, such as high efficiency and high packaging performance, etc.
During the eutectic bonding process, it often needs a bonding pad structure or a bonding ring structure. FIG. 1 illustrates an exemplary MEMS packaging structure.
As shown in FIG. 1, the MEMS packaging structure includes a MEMS device substrate 110, and a cover plate substrate 120. The MEMS device substrate 110 includes a passivation layer. Some parts of the MEMS device substrate are omitted in FIG. 1, and the passivation layer is not labeled. The passivation layer is partially etched to form trenches 112. The bottoms of the trenches 112 expose bonding pads 111. Protruding structures 121 are formed on the cover plate substrate 120. Further, a metal layer 122 is formed on the top surfaces of the protruding structures 121. During the packaging process, the metal layer 122 and the bonding pads 111 are fused together to form a metal alloy; and the metal layer 122 and the bonding pads 111 are bonded together. One of the purposes for forming the protruding structures 121 is to enable the metal layer 122 to reach the bottoms of the trenches 112 to contact with the bonding pads 111. One of the purposes to form the trenches 112 is to prevent the squeezing out and extending of the material of the bonding pads 111.
However, forming the protruding structures 121 and the trenches 112 increases process steps, process time and production cost, etc. Further, the protruding structures 121 are easily damaged; and the reliability of the packaging structure is reduced.
FIG. 2 illustrates an existing MEMS packaging method. As shown in FIG. 2, bonding pads 221 are formed on the bonding surface of the MEMS device wafer 210 directly without forming trenches. Then, bonding pads 221 are formed on the bonding surface of the cover plate wafer 220. Then, the bonding pads 211 and the bonding pads 221 are bonded together (not shown) to package the MEMS device wafer 210 and the cover plate wafer 220 together (not shown).
As shown in FIG. 3, the bonding pads 211 are made of one metal material. That is, the top view of the bonding pad 211 is a complete coverage structure. The structures of the bonding pads 221 are usually identical to the structures of the bonding pads 211. At least one of the metal material of the bonding pads 221 and the metal material of the bonding pads 211 has a relatively good ductility to facilitate the eutectic bonding process. When the bonding pads 211 having the complete coverage structure illustrated in FIG. 3 are used, it may be easy for the material having the relatively good ductility to be squeezed out and stretched during the bonding process. That is, during the bonding process, the material having the relatively good ductility overflows to the surrounding regions of the bonding pads 211. When the metal material overflows in the surrounding regions of the bonding pads with a relatively large amount, it may be easy to cause the adjacent conductive structures and the bonding pads to have a short-circuit issue. Accordingly, the reliability of the packaging structure may be reduced.
The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems in the art.